1. Field of the Invention
This invention relates to a method and apparatus for recording information signal on an optical disc and/or reproducing information signals from an optical disc.
2. Description, of the Related Art
Recently, optical discs have been used for recording/reproducing not only the conventional music and text information but also high-definition still pictures, speech or moving pictures of higher quality for a prolonged time. Consequently there is an increasing demand for an optical disc of larger recording capacity or an optical disc device of a smaller size exploiting a recording/reproducing technique of high recording density.
For increasing the information volume per unit area in an optical disc, there is known a method for reducing the shortest mark length recorded on the optical disc or narrowing the track pitch. However, it is necessary in this case to reduce the diameter of a light spot used for recording/reproducing the reduced shortest mark length.
As a method to realize this, research into a high density optical disc system in which the light spot diameter is reduced by increasing the numerical aperture NA of the objective lens used for converging the light in the optical disc device is proceeding briskly.
In a known manner, the diameter of a light spot produced by a collimated light beam falling on a lens is represented by the following equation (1): EQU W.sub.em =K.sub.w (.lambda./NA) (1).
The equation (1) states that the light spot diameter W.sub.em is obtained by multiplying a quotient of the light wavelength X by the numerical aperture NA or .lambda./NA with a constant K.sub.w determined over the shape of the lens aperture and the distribution of the intensity of the incident light beam.
Thus, the high recording density of the optical disc can be achieved by reducing the light spot diameter of the optical pickup by reducing the wavelength .lambda. of the laser light and increasing the numerical aperture NA.
If the numerical aperture of the optical pickup in the high recording density system is to be increased, and the diameter of the objective lens is the same as that of the conventional optical disc system, the working distance WD, which is the distance between the surface of the optical disc 100 and the objective lens 42, is shorter, as shown in FIG. 1.
That is, in FIG. 1A, a light beam 43 of the laser light having a diameter 2a is converged by an objective lens 42 spaced a working distance D from optical disc 100, so as to be illuminated through a medium of a refractive index n on a light converging point 41 on the optical disc 100 at a half angle .theta. of the maximum apex angle. The distance between the outer rim of the light beam 43 of the laser light on the objective lens 42 and the converging point 41 is R. The numerical aperture is given by the product of the refractive index n with the sine of the half angle .theta. of the maximum apex angle, or n.multidot.sin.theta.=n.multidot.a/R.
On the other hand, in FIG. 1B, a light beam 43 of the laser light having a diameter 2a is converged by the objective lens 42 spaced a working distance D' from the optical disc 100 so as to illuminated through a medium of a refractive index n on the light converging point 41 on the optical disc 100 at a half angle 6.theta.' of the maximum apex angle larger than the half angle .theta. of the above-mentioned maximum apex angle. The distance between the outer rim of the light beam 43 of the laser light on the objective lens 42 and the converging point 41 is R'. The numerical aperture is given by the product of the refractive index n with the sine of the half angle .theta.' of the maximum apex angle, or n.multidot.sin.theta.'=n.multidot.a/R'.
Comparison of FIGS. 1A and 1B indicates that, if the half angle .theta. of the maximum apex angle is increased to .theta.', the numerical aperture is increased from n.multidot.sin.theta.=n.multidot.a/R to n.multidot.sin.theta.'=n.multidot.a/R', however, the working distance is decreased from D to D'. Stated differently, the numerical aperture is increased, while the distance between the objective lens and the optical disc is decreased. Thus, if the maximum apex angle .theta. is increased to .theta.', the working distance D' is smaller than the working distance D, that is D&gt;D'.
The sequence of operations when applying a focussing servo in the routine optical disc device is explained with reference to FIGS. 2A to 2D.
First if the optical disc is set, the objective lens actuator moves in which the objective lens toward and away from the optical disc. In the actuator position shown at FIG. 2, the objective lens is at a position a furthest from the optical disc 100.
If the objective lens actuator causes the objective lens to be moved at a moderate speed in a direction approaching the optical disc 100, the reflection signals detected by a photodetector are increased progressively. At a time point when the focal length between the objective lens and the optical disc is less then a few .mu.m, a negative signal starts to be generated for the focussing error signal shown in FIG. 2B. The position b of the actuator shown in FIG. 2A corresponds to the time point the negative focussing error signals start to be produced.
If, with the use of, for example, the astigmatic method, the objective lens is moved further closer to the optical disc, the focussing error signal, shown in FIG. 2B, reaches a locally maximum value, and subsequently starts to decrease. From this time on, the focussing error signals and the distance between the objective lens and the optical disc coincide with each other. Thus, when the reflected light volume signal exceeds a predetermined level, the focussing error signals are detected and the focussing servo loop is turned on when this signal reaches 0, in order to pull in the focussing servo.
At a position the focussing error signals reach zero, the reflection light volume detection signal, shown in FIG. 2C, becomes maximum. This position corresponding to the maximum reflection light volume detection signal corresponds to an actuator position c shown in FIG. 2A.
If the focussing servo has been pulled-in successfully, the feedback loop of the focussing servo including the focussing error signals is in operation. Thus, the objective lens is driven by the objective lens actuator so that the focussing error signals will be substantially equal to zero. In FIG. 2, the behavior of the respective signals is as shown by a solid line. This sequence of operations is termed the focussing search operation.
Among the family of recordable optical discs, there is a disc employing a phase change material. A mark which serves as data is produced by forming a crystal area and an amorphous area by controlling the laser light power. The reflectance of the crystal area is approximately 20%, while that of the amorphous area is approximately 0.6%. The mark can be discerned by this difference in reflectance. The average data surface reflectance is of the order of approximately 10% which is not vitally different from the surface reflectance of the optical disc which is approximately 6 to 7%.
If the objective lens is actuated in order to pull in the focussing servo, two similar reflection and focussing error signals are produced. For focussing pull-in on the data surface, there is required a method to make distinction between these two signals. If the disc surface of the optical disc is close to the signal recording surface and the focussing servo pull-in has failed, there is a risk of the objective lens colliding against the disc surface.
If the objective lens is actuated from the surface towards the data surface in order to detect the focussing servo pull-in range, there may be contemplated a method of detecting the second focussing error signal.
However, if the optical disc surface is scratched or dust and dirt are deposited thereon, signals cannot be detected correctly, such that the discrimination and hence the focussing servo pull-in results in failure to lead to collision of the optical disc against the objective lens as in the case of the above-mentioned failure in focussing servo pull-in. On the other hand, if vibrations are applied to the device from outside, such that the objective lens undergoes vibrations in the vicinity of the optical disc surface, signal chattering is produced to lead to mistaken focussing servo pull-in on the optical disc surface.